Shielded stranded pair



March 17, 936. E. I. GREEN ET AL ,0

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ATTORNEY Patented Mar. 17, 1936 UNITED STATES SHIELDED STRANDED PAIREstill I. Green andlHaroldE. Curtis, East Orange, N. J., assignors toAmerican Telephone and Telegraph Company, a corporation of New York.Application June 7, 1933, Serial No. 674,763 l Claims. (Cl. 17844) This.invention relates to electrical transmis- ISlOIl circuits and isconcerned especially with circuits :comprising a pair of conductorssurroundedby an individual shield. A particular 5 object of theinvention is to obtain an individually shielded circuit which has theproperties of low attenuation and :substantial freedom from externalinduction throughout a wide range of frequencies. Another object oftheinvention is -to-obtaina circuit of such characteristics which isbalanced with respect to ground.

The frequency :range which 'may be transmitted over a. circuitconsisting of :an ordinary unshielded paira'of conductors is limitedboth by the increasing "susceptibility of the circuit to crosstalk fromnearby conductors andrinterference from external-sources as thefrequencyis. increased, and-also, in "many instances, by thelarge highrfrequencyattenuation-which results from thezuse of' solid dielectric material. Inaccordance with the invention it is proposed to-enclose aapairof'conductorsina conducting shield which acts topreventexternal:electromagneticor electro-static disturbances fromcausing disturbances in thepainandconversely, to prevent the currentstransmitted over thepair from causing disturbances in external circuits.Moreover, since the shielding effect of such an enclosing:shielddecreases as the vfrequency*decreases, it is proposedinaccordance with the invention to twist the conductors-ofthe pairhelically about the axis of the shield or otherwise transpose them inordertto annul any-interference which may .pass through the shield atlow frequencies.

1 In order to reduce the high frequency attenuation of the shielded pairit is proposed in one em- :bodiment of theinvention to employ. asubstantially-gaseous dielectric between the conductors of the pair andbetween these conductors and thecsurrounding sheath. The inventioncomprehends :also,..however, the use of non-gaseous dielectricmaterialto insulate the conductors from oneanotherand from the sheath.

.Theinvention has to do especially with individually shieldedpairs'ofconductors in which each conductor is constituted of a' number ofinsulatedstrands or filaments which are so interwoven or arranged as toreduce the high frequency resistanceof the circuit. Pairs of conductorswhich are surrounded by-shields of substantially circular cross-sectionare a particular subject of the invention.

Aparticular object of the invention is to so proportion the ratio of theinner diameter of the H5 shield to the diameter of the conductorsandcthe ratio of the interaxial separation between conductors to theinner diameter of'the shield that, for. any given degree of.stranding'and for a given size ofshield, the attenuation 'willbe aminimum.

,Morebroadly, the invention is concerned with systems for utilizingindividually shielded balanced pairs for the transmission of highfrequencies or wide bands of frequencies. Satisfactory transmission oftelevision images with good definition requires the transmission of afrequency band which may extend from zero frequency to hundreds or eventhousands of kilocycles. If, for example, it is desired to transmit witha total of 24 reproductions per second an image containing 40,000picture elements, there is required a frequency band of approximately500 kilocycles in width. Still wider bands may be necessary forrepresenting with adequate detail such scenes as a theatricalperformance or an athleticevent. A shielded transposed pair designed inaccordance with the principles of the invention is especially suited forthe transmission of such television bands because it may begivencomparatively low attenuation and relative freedom frominterference over the entire band.

Moreover, by the application of multiplexing the wide frequency bandsobtainable from the shielded transposed pair may be used to providesubstantial numbers of narrower frequency bands suitable for other uses,as for example, for telephone circuits which may require bands of about2500 cycles in width, for high quality program circuits which mayrequire bands extending up to 10,000 cycles or higher, for high speedfacsimile transmission, or for other purposes.

Inasmuch as the two conductors are symmetrical at allpoints with respectto the shield, the potential between each conductorand the shield isequal. Therefore, if such a shielded stranded pair were buried so thatthe shield makes electrical contact with the ground, or if the shieldwere electrically connected to ground at frequent intervals, the twoconductors would form a balanced-to-ground circuit. Even if the shieldwere not connected by wires to ground or buried, it would be-efiectivelyconnected to ground due to the electrical capacity between itand ground.Hence, the two conductors will always form a balanced-to-ground circuit.Such a balancedto-ground circuit would be very useful forinterconnecting electrical elements which are themselves balanced toground.

For instance, it is frequently desirable in the radio art to employ anantenna which is balanced with respect to ground rather than to connect,transmit or receive between antenna and ground. Such, for example, isthe case when using adiamond antenna or a horizontal dipole antenna. Ashielded twisted pair of the type described herein is peculiarly adaptedfor connecting such balanced antennas with radio transmitting orreceiving apparatus inasmuch as such a pair may be balanced to groundand may be designed to have low attenuation and substantial immunityfrom external interference at the frequency or frequencies employed forradio transmission.

These and other objects and features of the invention'will be morereadily understood from the following description when read inconnection.

with the accompanying drawings, in which Fig. 1A, 1B and 1C representvarious transmission systems utilizing shielded stranded pairs; Figs,

2, 3, 4, 5 and 6 are sections of various shielded stranded pairs; Fig. 7is a view of a stranded conductor; Fig. 8 shows the ratio of theresistance of a stranded conductor to the resistance of a solidconductor of the same diameter versus frequency for two conditions ofstranding Fig. 9 is a section of a shielded stranded pair; Figs. 10 and'11 are curves indicating the optimum proportioning of a shieldedstranded pair to obtain minimum high frequency attenuation; and Fig. 12shows the characteristic impedance of a shielded pair having optimumproportioning for high frequency transmission.

Overall systems embodying the invention are schematically illustrated inFigs. 1A, 7B and 1C. In these figures shielded pair transmission linesare shown associated with various kinds of appa ratus at the terminals.Thus in Fig. 1A is shown line used to connect a radio transmitter RT1 toa balanced-to-ground radio antenna RTz, for example, a horizontaldipole. If desired, the shield can be buried as shown in Fig. 10.

Referring to Fig. 2, I and 2 represent two conductors formed ofinsulated strands, said conductors being held in proper relation and outof electrical contact wtih each other and with the circular conductingshield 3 by means of spaced dielectric washers 4. These washers shouldbe separated from each other a suitable distance and should bemade asthin as possible consistent with the required mechanical strength. Theyshould also be composed of some dielectric material of small loss angleand low dielectric constant, for if these conditions are obtained, theleakage loss may be made so small as to be practically negligible. Fig.3 shows a longitudinal section through a shielded stranded pair havingsolid dielectric; l and 2 are the two conductors formed of insulatedstrands, 3 is a circular shield composed of a number of insulatedstrands, and 3 represents the circular conducting shield. In

'Fig. 4, 0 represents the inner radius of the shield, id one-half theinteraxial separation of the two stranded conductors l and 2, and 2)represents the radiusof the conductors. Figs. 5 and 6 are longitudinalsecjfpns of a shielded pair} I and 2 are conductors composed of a numberof insulated strands twisted so as to form a conductor of an nularcross-section, the core of the conductors being filled with somenon-conducting material as jute or a conducting material such as copperor steel. 3 is the conducting shield in the form of' a tube composed ofoverlapping tapes of Z- shaped cross-section. The dielectric 4 in Fig. 5is in the form of thin insulating washers; that in Fig. 6 is a soliddielectric such as paragutta.

The conductors may consist of a number of? strands, filaments, tapes, orthe like, which are: insulated from one another and are interwoven orbraided together in any of various ways. Ordinarily the purpose of suchstranding would be to reduce the resistance of .theconductor's at high.frequencies by counteractingthe tendency of the currents to concentrateon the surface of the con.- ductor and thereby to decrease'the highfrequency attenuation of the circuit. .Another important result obtainedby stranding, however, is an increase in the internal'inductance of eachconductor which'likewise reduces the high frequency attenuation.Stranding may also be advantageous from the standpoint of obtaining aflexible structure.

In order to counteract the tendency of the currents to concentrate onthe surface of the conductor at high frequencies, it is essential thatthe insulated strands be passed back and forth to=- ward and from thecenter of the conductor. With a suitable method of stranding, the highfrequency current may be distributed substantially uniformly over thecross-section of the conductor. One method of obtaining this result is'to strand the conductor in a manner similar to that used in themanufacture of rope. Thus, several individual strands (for example,three) would first be twisted together, next several of these groupswould be twisted together to form larger groups, and several ofthelarger groups would be twisted together, the process being continueduntil the desired total number of strands is obtained. If the strandinginterval or pitch is made different for the successive twistingoperations, it will be found that with such a procedure any one strandin going along the conductor travels a path back and forth between thecenter of the conductor and its periphery. A stranded conductor built upin this manner is illustrated in Fig. 7.

Instead of twisting the strands as described above they might beinterwoven or braided in other ways so as to produce the same effect.Also, it would be possible to employ an annular crosssection for theinsulated strands, thecore of the conductor being filled up with somenon-conducting material such as jute, or with a conducting material suchas copper or steel to provide rigidity or strength as shown in Figs. 5and 6. The stranding might be arranged in such a' way that the path ofany strand would extend between the outer and inner circumferences ofthe annulus.

The two conductors may be either parallel or transposed at frequentintervals. One method of transposition would be to twist the twoconductors helically about the axis of the shield. As will be broughtout below, the optimum proportioning of the circuit will be the'same ineither case.

For the insulation between the two conductors and between conductors andsheath, any'of various forms or shapes may be employed. One possiblearrangement would be to use a continuous spirally applied string orstrip 0f dielectric -material. Generally, it will bedesirable .thattheamount of insulating material employed be a. minimum in order that thedielectric between the two conductors may be largely gaseous. 'In someinstances, however, it may be found. advantageous to use a dielectricwhich is. partly or wholly nongaseous, as, for example, rubberinsulation, as illustrated in Figs.'3 and 6.

The shield surroundingthe two conductors, instead of being formed of asingle tube, might consist of a cylindrical assembly of conductingstrips, tapes, ribbons, wires, or the like. .Such forms of constructionmight be particularly advantageous where a flexible structure isdesired.

In connection with the shield it may be noted that in addition toperforming an electrical function by protecting the circuit fromexternal induction, it may be useful in affording mechanical protectionto the circuit and thereby permitting the use of air dielectric to avery considerable extent. Due to skin effect the high-frequency currentswill penetrate very little into the shield so that the electricalrequirements are satisfied by a very thin shield. Consequently, thethickness of the shield will ordinarily be determinedby-mechanicalconsiderations. The thickness of the shield will usually besuch that itdoes not enter into the problem of determining the optimumconfiguration of conductors and shield.

The use of the shield will ordinarily make it possible where desired toallow the signals transmitted over the pair to drop down to a minimumlevel determined by the noise due to thermal agitation of electricity inthe conductors. Hence the use of the shield may facilitate the spacingof intermediate amplifiers in the circuit at wider intervals than wouldotherwise be possible.

The configuration of the oonductors andshield which results in. minimumhigh frequency attenuation can be determined by obtaining'anexpressionfor the attenuation of the system and then determining the diameter andspacing ratios which make the attenuation a minimum for any given innerdiameter of the shield. It will be assumed that the'frequency is abovethe audible range and that the leakage is zero (a condition that may beapproximately obtained)- All units are in the c. g. s. electromagneticsystem.

The attenuation at high frequencies of an electrically smoothtransmission line with zero leakage is given very closely by theexpression where R represents the resistance, C1 the capacitance and Lthe inductance of the system. The total resistance R of the system ismade up of the resistance of the stranded conductors with the shieldabsent and the resistance due to eddy current loss in the conductingsheath, proximity effect between the two conductors being eliminated bystranding. The high frequency resistance (in abohms per centimeter) ofthe two stranded conductors without the shield may be written as 1)being the radius of the conductors, f the .frequency in cycles, A theconductivity of the shield material (approximately 5.8 i0 abohms per cm.cube for copper), and n the ratio of the resistance at the samefrequency of the stranded conductor to the resistance of a solidconductor of the same outer diameter but of aconductivity Royal Societyof London,

.equal to that of the shield. The value of n for a conductor that iscompletely stranded, i. e. so

tained froma formula by lished in the Philosophical S. Butterworth pub-Transactions of the vol. 222, page 57.

Equation therein should be modified by the omission of the two termswhich involve D when it will read This gives the A. C. resistance of acompletely stranded conductor in abohms per centimeter. The meanings ofthe various symbols and the reason for omission of the two termsmentioned abovewill be found in the article. The high frequencyresistance R1 of a solid wire (in abohms per centimeter) is given by theexpression ii bx The value of n can be determined by dividing R by R1.Fig. 8 shows how 11. varies with frequency for two conditions ofstranding when all of the conductors are of copper.

For any completely "stranded conductor there is ordinarily a frequencyat which the ratio 71 is a minimum. The lowest value of n that can beobtained for a completely stranded conductor of a given diameter'andsize of strands together with the number of strands and the thickness ofinsulation necessary to produce this value of n can be determined byExpression 1. The conductors should be designed so as to have as low avalue of n as'possible at the given frequency, and the sizeandseparation of the conductors should be determined for this minimumvalue n if it is desired to obtain as low an attenuation as possible atagiven frequency for a shield of given diameter and material,

If the conductors are not completely stranded, the value of n'may bedetermined either by computation or experiment.

The increment in the resistance of the two stranded conductors due tothe current in the conducting sheath may be obtained in the followingmanner. Referring to Fig. 9, let points A and B represent the centers ofthe two stranded wires each spaced a distance d from E which is thecenter of the sheath having a radius 0. The effect of the sheath may bereplaced by the two image Wires A and B, each spaced a distance from Eas described on page 1'74, Radio Frequency Measurements (i931), E.'B.Moullin.

From a consideration of the magnetic fields due to the conductors andtheir images, the tangential magnetic field intensity at any point P onthe sheath can be shown to be 0 (1 20d cos 0 where H is the magneticfield intensity, I is the currentzin the conductors and 0 the angle PEB(Fig. 9). The normal component at P is zero. At high frequencies thefield outside the shield is substantially zero, so we can assume acurrent in the shield producing a field equal and opposite that due tothe conductors and their images.

Therefore the current densityN at P equalsCzI-li where C2 is a constant.

In a given material the power loss in the sheath is proportional to Nand to the area; .thus the power loss in the sheath per unit length isWhere W=p-ower loss in the sheath per unit length Ca constant, c=radiusof sheath.

Into this expression for the power loss we can place the expression forthe field H at the point P: thus W1 0 c +d 2cd cos 0 (4) 1 c +d +2cd cos0 (10 This becomes:

cd C C W 641 11" (5) By the same .method we may obtain the power loss inthe circular sheath of radius 0 when used as a concentric return for awire placed in its center. Thus the field 2I H =-c The current densityN1 is proportional to H1. For the same frequency and material as aboveThe high frequency resistance R2 of the shield when used as a concentricreturn for an inner conductor is given by the well known expressionTherefore the total resistance R of the shield pair is f n 4011 5 c dThe capacity between two completely stranded wires when surrounded by acylindrical metallic sheath. is given by the expression:

The inductance of the system is made up of the inductance due to themagnetic field outside the conductors and the internal inductance due tothe flux within the conductors. The external .inductance equals therciprocal of the capacity of the circuit multiplied by the dielectricconstant K. The internal inductance is a function of the distribution ofthe flux in the conductors. For stranded conductors the internalinductance at high frequencies is approximately that at direct currentin so much as the current distribution is relatively independent offrequency up to very high frequencies. The internal inductance of a pairof completely stranded conductors is then one aosgoas abohmpercentimeter. The total inductance be- Therefore the attenuation at highfrequencies and for zero leakage is Where d2 22! 1 b d 1 g On imposingthe first condition we find that Imposing the second condition we findthat 2 1Og: (4 log M+ 1) 1 Combining the left-hand members of Equations(17) and (18) and substituting the values of the derivatives, thefollowing expression results This expression is the locus of values ofthe diameter ratio which give minimum attenuation for different assumedvalues of the wire spacing ratio O The unique values of d o z and whichgive minimum attenuation for a given resistance ratio n are obtained bytaking pairs of c andwhich satisfy Equation (19) and substituting themin Expression (13) for the attenuation and graphically determining thepair that gives minimum attenuation. I

Fig. 10 shows graphically the relation tha should exist between thespacing ratio and the resistance ratio n for minimum attenuation for anypredetermined inner diameter of shield. The relationship between and nas indicated by the curve of Fig. 10 can be approximated closely by' theempirical expression The relationship that should exist between and nfor minimum attenuation for any predeter-' It can be seen. moreoverFigs. 10 and 11 thatfor between about 0.5 and 1.11

can be approximated roughlyby 0.40 and by 5.0 irrespective of: the valueof 11.

However, ifit' is desired to design a shielded stranded pair so that theattenuation will be more exactly a minimum for any particular value ofn, the relations given in Figs. 10 and 11 or the equivalent empiricalexpressions given above should be used.

It must be borne in mind that the value of n tobe used in designing ashielded stranded pair is the value that is given by the maximumfrequency to be transmitted over the transmission line. If any givenshielded stranded pair is designed in the above manner, the attenuationat the maximum frequency will be as low as possible at this frequency.At any lower frequency the system will not have the optimum design butsince the attenuation is less at the lower frequencies, this fact doesnot matter. The same value of 12 determines both-ratios by examinationof the" values of'n lying thev ratio The high frequency impedance ofsuch a conductor system is given closely by the expressionHence'proportioning the conductors in such a manner as to obtain minimumhigh frequency attenuation as disclosed above results in a certain highfrequency characteristic impedance corresponding to each value of n.This relation between the characteristic impedance and n is Example Theproportioning of shielded pair circuits so as to secure with a givensize shield the lowest possible attenuation at'a given maximum frequencyis complicated slightly by the fact that the size of the conductors fora'given diameter of shield depends on the value of n, and the lowestvalue of n attainable at a given frequency depends upon the size of theconductor as Well as other quantities. Hence the method of designingsuch a circuit is one of successive approximations; To illustrate thismethod a simple example will be considered. Thus it may be desired to soproportion a shielded pair circuit of copper whose shield is to be .500"in inner diameter and Whose enclosed conductors are to be completelystranded and composed of No.- 40 B. & S. gauge strands, that theattenuation at 300 kc. will be as low as possible. From Fig. 11 it canbe determined that the conductor should be approximately /5 the diameterof the shield, i. e.', .100" in diameter. From Expression (1) it can-bedetermined that the lowest value of n attainable for this diameter underthe'above conditions is .46. Considering Fig. 11 again'it is seen thatthe diameter ratio should have been more exactly 54; hence the conductordiameter becomes For this sized conductor the lowest-value of n for thegiven conditions of frequency and strand size is .47.

From Fig. 11 it is seen that the optimum diameter ratio-changes littlefrom n=0.46 to n=0.47. Hence theconductor diameter shouldbeapproximately .093". From a consideration of Expression (1) it can bedetermined that the minimum value of n for a .093 conductor composed'ofNo. 40 B; & S. strands is obtainable by using approximately-500 strands,each strand having.

the radius of thean insulation thickness 20% strand. From Fig. it canbe'determined that the ratio of interaxial separation of the conductorsto the inner diameter of the shield should be approximately .365. Hencethe interaxial separation'of the conductors should be approximately0.500" .365=.182". a circuit for any other size shield, maximumfrequency and size of strand can be carried out similarly.

The attenuation of shielded pair circuit as proportioned in the previousparagraphs can be obtained from Expression (13). Substituting numericalvalues in the expression and changing from c. g. s. absolute units topractical units we The design of such where L0 is the internalinductance of the two conductors in abhenries per centimeter. ThereforeExpression (13) for the attenuation of the 15 system becomes a It isdifiicult to obtain a general solution of this problem, i. e., todetermine the value of i and which gives minimum attenuation for anyvalue of n and L0, because the internal inductance of the conductorsdepends upon the cross-section of the conductors. For example, if theconductors are annular in cross-section, the internal inductance of thetwo conductors is given by the expression +1) 2 2 EZT-TFFF W where b isthe outer radius and a is the inner radius of the stranded annularconductor. For other stranded conductors the internal inductance can bedetermined either experimentally or by pub- ,1ished formulas. Thecorrect proportioning for minimum attenuation for any frequency andconductor cross-section can be determined by putting the correspondingvalues of L0 and 11. into Expression (20) anddetermining graphically thevalues of g and g which makes it a minimum for a predetermined avalue ofc.

The high frequency characteristic impedance of a shielded pair system inwhich the conductors are not completely stranded is approximatelywhereas before M equals In the derivation of the ratios conductors andshield in their proper relationship, the insertion of a certain amountof dielectric inside the shield would be necessary. By using as small anamount of dielectric as possible and by using dielectric material with asmall dielectric constant and low power factor, the eifect of thedielectric on the constants of the line could be made negligible.Accordingly, the diameter ratios and spacing ratios for various valuesof 11. would remain as disclosed.

It is possible however, to proportion a shielded pair of predeterminedsize so that it will have minimum attenuation at any given value of neven though the dielectric loss is large, provided that the averagedielectric constant is independent of the proportions of the circuit.

The high frequency attenuation of a smooth line having leakage is givenby the expression Equations (11), (12) and (13),Equation (22) reduces to1 where, as before,

w=conductivity in abmho per centimeter cube and c=inner radius of shieldin centimeters. From Equation (23) it is evident that the diameter ratioand spacing ratio that will give minimum attenuation depend on n, P, f,c and A. By assuming various values for P, f, c and A, the values of c d3 and 6 can be determined graphically for any value of n which will makethe attenuation as given by Equation (23) a minimum. Doing this forvarious combinations of P, c and A, it is possible to determine thedesired relation between and n for various values of P, f,-c and A. Bycomparing these values of.

3 f 2 to those obtained for wholly gaseous dielectric, it is possible toobtain an empirical relation between the values of $084,033 whichobtains fora system having no dielectric loss and those for systemshaving appreciable dielectric loss. Thus, it can be shown graphicallythat the value of is not affected appreciably by the leakage loss in thedielectric. However, the value of to be used for any frequency, powerfactor and diameter of shield can be obtained approximately bymultiplying the value of a very low amount the resultant disturbance inthe circuit due to inadequate shielding. The pair is shielded at highfrequencies from outside disturbances and also does not produce anydisturbance outside the sheath due to the shielding effect produced bythe eddy currents induced in the shield. Thus, a shielded transposedpair is relatively free from external interference at all frequenciesand hence is useful and advantageous for the transmission of frequencybands extending down to zero frequency.

The condition that must be satisfied for maximum high frequencycharacteristic impedance for a shielded stranded pair can also be detercmined The high frequency characteristic im- 3 pedance of a shieldedstranded pair is:

d 2 2e d 3 20 d F z 410g. X- o g. 2 4 b 0 3 b c 1 21 for zero leakage bythe expression for values of 11 less than 1.1.

Thus, for n less than 1.1 the value of b for any power factor, frequencyand diameter shield for copper conductors is given approximately by-theexpression where e is the diameter ratio for the case of negligibleleakage as shown in Fig. 7. For 12 greater than 1.1,

b should be approximately 5;

O isglven approximately by the expression regardless of f, P, A and c.

Inasmuch as the high frequency characteristic impedance of atransmission system is relatively independent of the dielectric loss,the characteristic impedance of a shielded pair system having dielectricloss is equal approximately to the air dielectric characteristicimpedance divided by the square root of the average dielectric constantof the insulating medium.

It has been assumed that the two stranded conductors are either parallelor transposed at frequent intervals, maintaining at all points theproper diameter and spacing ratios, except, however, at thetransposition points. In the case of the helically twisted type oftransposition, the twisting of the conductors does, not materiallyaffect the conditions for minimum attenuation providing the twist isrelatively long compared with the diameter of the conductors.

At low frequencies at which. the shield does not provide adequateelectromagnetic shielding, the

transposing of the conductors serves to reduce to.

The internal inductance L0 being independent of the diameter ratio trioand the spacing ratio the ratio for maximum characteristic impedance forany given diameter ratio and putting this derivative equal to zero. wefind that Thus for maximum characteristic given ratio of inner diametereter of conductor.

It will be obvious that the general principles herein disclosed may beembodied in many other organizations widely different from thoseillustrated without departing from the spirit of the invention asdefined in the following claims.

What is claimed is:

1. A transmission circuit comprising two cylindrical conductors ofsubstantially the same impedance for any of shield to diamsize arrangedside by side and spaced apart, a.

cylindrical conducting shield surrounding said conductors, one of saidconductors being connected as a return for the other to form a, highfrequency transmission path shielded. by said shield, said conductorsand shield being insulated from one another by a substantially. gaseousdielectric, each of said conductors consisting of a plurality ofconducting strands insulatedv from one another and so stranded thatcurrent distribution is substantially uniform for a range of frequenciesextending to a region well above audibility, the transmission pathformed from said cylindrical conductors acting one as a return for theother, having connected thereto apparatus for applying thereto andreceiving and utilizing therefrom a band of signal frequencies extendingfrom approximately zero to a frequency many times the upper limit of theaudible range, said path with its associated shield acting to transmitwithout excessive attenuation the band of frequencies so applied.

2. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size arranged side by side and spaced apart, acylindrical conducting shield surrounding said conductors, one of saidconductors being connected as a return for the other to form a highfrequency transmission path shielded by said shield, said conductors andshield being insulated from one another by a substantially gaseousdielectric, each of said conductors consisting of a plurality ofconducting strands insulated from one another and so stranded thatcurrent distribution is substantially uniform for a range of frequenciesextending to a region well above audibility, the transmission pathformed from said cylindrical conductors acting one as a return for theother having connected thereto apparatus for supplying thereto andreceiving and utilizing therefrom a band of signal frequencies extendingfrom approximately zero to a frequency many times the upper limit of theaudible range, said path with its associated shield acting to transmitwithout excessive attenuation the band of frequencies so applied.

3. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size arranged side by side and spaced apart, eachof said conductors consisting of a plurality of conducting strandsinsulated from one another and so stranded that current distribution issubstantially uniform for a range of frequencies extending to a regionwell above audibility, a cylindrical conducting shield of predetermineddiameter surrounding said conductors, one of said conductors beingconnected as a return for the other, said conductors and shield beinginsulated from one another, the relative dimensions of said conductorsand shield and their relative spacing being such that the high frequencyattenuation of the circuit will be a minimum for any frequency where theratio of the resistance of either of said conductors to the resistanceat the said frequency of a solid conductor of the same material and sizeis less than unity.

4. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size arranged side by side and spaced apart, eachof said conductors consisting of a plurality of conducting strandsinsulated from one another and so stranded that current distribution issubstantially uniform for a range of frequencies extending to a regionwell above audibility, and a cylindrical conducting shield ofpredetermined diameter surrounding said conductors, one of saidconductors being connected as a return for the other, said conductorsand shield being insulated from one another by a substantially gaseousdielectric, the relative dimensions of said conductors and shield andtheir spacings being such that the high frequency attenuation of thecircuit will be a minimum at any frequency for which the ratio of theresistance'of either of'said' conductors to the resistance at the samefre quency of a solid conductor of the same material and size liesbetween the limits unity and 0.4.

5. A transmission circuit comp-rising two cylindrical conductors ofsubstantially the same size arranged side by side and spaced apart, eachof said conductors consisting of a plurality of conducting strandsinsulated from one another and so stranded that current distribution issubstantially uniform for a range of frequencies extending to a regionwell above audibility, and a cylindrical conducting shield ofpredetermined diameter surrounding said conductors, one of saidconductors being connected as a return for the other, said conductorsand shield being insulated from one another by a substantiallynongaseous dielectric, the relative dimensions of said conductors andshield and their relative spacings being such that the high frequencyattenuation of the circuit will be a minimum for the frequency for whichthe ratio of the resistance of either of said conductors to theresistance at the same frequency of a solid conductor of the samematerial and size is substantially a minimum.

6. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size arranged side by side and spaced apart, eachof said conductors consisting of a plurality of conducting strandsinsulated from one another and so stranded that current distribution issubstantially uniform for a range of frequencies extending to a regionwell above audibility, and a cylindrical conducting shield ofpredetermined diameter surrounding said conductors, said conductorsbeing helically twisted around the axis of said shield, one ofsaidconductors being connected as a return for the other, said conductorsand.

shield being insulated from one another, the relative dimensions ofthesaid conductors and shield and their relative spacings being suchthat the high frequency attenuation of the circuit will be a minimum atany frequency'for which the ratio of the resistance of either ofsaidconductors to the resistance at the same frequency of a solidconductor of the same material and size lies between the limits unityand 0.4.

7. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size arranged side by side and spaced apart, eachof said conductors consisting of a plurality of conducting strandsinsulated from one another and so stranded that current distribution issubstantially uniform for a range of frequencies extending to a regionwell above audibility, and a cylindrical conducting shield ofpredetermined diameter surrounding said conductors, said conductorsbeing transposed about the axis of the shield, one

of said conductors being connected as a return the same material andsize is substantially a minimum.

8. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size arranged side by. side and spaced apart, acylindrical conducting shield of predetermined diameter surrounding saidconductors, one of said conductors being connected as a return for theother, saidconductors and shield being insulated'from one another, eachof said conductors consisting of a plurality of conducting strandsinsulated from one another and so stranded that current distribution issubstantially uniform for a range of frequencies extending to a regionwell above audibility, the particular arrangement of said strands beingsuch as to produce at a given high frequency a predetermined value forthe ratio of the resistance of each of said conductors to the resistanceof a solid conductor of equivalent diameter, the relative dimension ofsaid conductors and shield and their relative spacings being such thatthe high frequency attenuation of the circuit will be a minimum at saidgiven high frequency.

9. A transmission circuit comprising two conductors, a cylindricalconducting shield surrounding said conductors, one of said conductorsbeing connected as a return for the other, said conductors and shieldbeing insulated from one another, each of said conductors consisting ofa plurality of conducting strands insulated one from another, the ratioof the interaxial separation of said conductors to the inner diameter ofsaid shield being approximately .40 and the ratio of the inner diameterof said shield to the diameter of each of said conductors beingapproximately 5.0.

10. A transmission circuit comprising two conductors, each of saidconductors consisting of a plurality of conducting strands insulatedfrom one another, and a cylindrical conducting shield surrounding saidconductors, one of said conductors being connected as a return for theother, said conductors and shield being insulated from one another by asubstantially gaseous dielectric, the ratio of the interaxial separationof said conductors to the inner diameter of said shield beingapproximately .40 and the ratio of the inner di ameter of said shield tothe diameter of each of said conductors being approximately 5.0.

11. A transmission circuit comprising two conductors, each of saidconductors consisting of a plurality of conducting strands insulatedfrom one another, and a cylindrical conducting shield surrounding saidconductors, one of said conductors being connected as a return for theother, said conductor and shield being insulated from one another by asubstantially non-gaseous dielectric, the ratio of the interaxialspacing of said conductors to the inner diameter of said shield beingapproximately .40, and the ratio of the inner diameter of said shield tothe diameter of each of said conductors being given approximately by theexpression:

where n is the ratio of the resistance of each of said conductors to theresistance of a solid conductor of the same diameter at the frequency F,F is the frequency in cycles per second, A the conductivity in abmho percentimeter cube, p the power factor, and c is the inner radius of theshield in centimeters.

12. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size arranged side by side and spaced apart, eachof said conductors consisting of a plurality of conducting strandsinsulated from one another and so stranded that current distribution issubstantially uniform for a range of frequencies extending to a regionwell above audibility, and a cylindrical conducting shield ofpredetermined diameter surrounding said conductors, one of saidconductors being connected as a return for the other, said conductorsand shield being insulated from one another, the relative dimensions ofsaid conductors and shield and their spacing being such that the highfrequency characteristic impedance of the circuit derived from saidstranded conductors will approximate M3 ohms for the usual value of n.

13. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size arranged side by side and spaced apart, eachof said conductors consisting of a plurality of conducting strandsinsulated from one another and so stranded that current distribution issubstantially uniform for a range of frequencies extending to a regionwell above audibility, and a cylindrical conducting shield ofpredetermined diameter surrounding said conductors, one of saidconductors being connected as a return for the other, said conductorsand shield being insulated from one another by a substantially gaseousdielectric, the relative dimensions of said conductors and shield andtheir spacing being such that the high frequency characteristicimpedance of the circuit derived from said stranded conductors willapproximate 143 ohms for the usual value of n.

14. A transmission circuit comprising two conductors, each of saidconductors consisting of a plurality of conducting strands insulatedfrom one another, and a cylindrical conducting shield surrounding saidconductors, one of said conductors being connected as a return for theother, said conductors and shield being insulated from one another, theratio of the interaxial separation of said conductors to the innerdiameter of said shield being approximately .486.

15. A transmission circuit comprising two cylindrical conductors ofsubstantially the same size, the interaxial separation of saidconductors being greater than the diameters, a cylindrical conductingshield of predetermined diameter surrounding said conductors andsubstantially concentric with a line midway between the centers of thetwo said conductors, one of said conductors being connected as a returnfor the other to form a high frequency transmission path shielded bysaid shield, said conductors and shield being insulated from oneanother, each of said conductors consisting of a plurality of conductingstrands insulated from one another and so stranded that currentdistribution is substantially uniform for a range of frequenciesextending to a region well above audibility, the transmission pathformed from said cylindrical conductors acting one as a return for theother having connected thereto apparatus for applying thereto andreceiving and utilizing therefrom a band of signal frequencies extendingfrom approximately zero to a frequency many times the upper limit of theaudible range, said path with its associated shield acting to transmitwithout excessive attenuation a band of frequencies so applied, therelative dimensions of said conductors and shield and their relativespacings being such that the hi h frequency attenuation of the circuitwill be a minimum at the highest frequency of said band of signalfrequencies.

ESTILL I. GREEN. HAROLD E. CURTIS.

